Raman Spectroscopy for Chemical Analysis-J.D.Winefordner

Raman Spectroscopy for Chemical Analysis CHEMICAL ANALYSIS A SERIES OF MONOGRAPHS OF ANALYTICAL CHEMISTRY AND ITS APPLICATIONS Editor J D WINEFORDNER VOLUME 157 A JOHN WILEY & SONS, INC., PUBLICATION New York I Chichester I Weinheim I Brisbane I Singapore I Toronto Raman Spectroscopy for Chemica1 Analysis RICHARD L McCREERY The Ohio State University Columbus, Ohio A JOHN WILEY & SONS, INC., PUBLICATION New York I Chichester i Weinheim I Brisbane I Singapore I Toronto This book is printed on acid-free paper @ Copyright 2000 by John Wiley & Sons, Inc All rights reserved Published simultaneously in Canada N o part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise except as permitted under Sections 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, 222 Rosewood Drive, Danvers MA 01923, (978) 750-8400, fax (978) 750-4744 Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 605 Third Avenue, New York, N Y 10158-0012, (212) 850.601 I , fax (212) 850-6008, E-mail: PERMRBQ@WILEY.COM For ordering and customer service, call I -800-CALL-WILEY Library of Congress Cataloging-in-PublicationData: McCreery, Richard L Raman spectroscopy for chemical analysis / by Richard L McCreery p cm.-(Chemical analysis ; v 157) "A Wiley-interscience publication Includes index ISBN 0-471-25287-5 (alk paper) I Raman spectroscopy Chemistry, Analytic I Title IT Series QC454.R36.M33 2000 543'.08584-dc21 99-08649 Printed in the United States of America I09 CONTENTS PREFACE ix ACKNOWLEDGMENTS xiii LIST OF SYMBOLS xv CUMULATIVE LISTING OF VOLUMES IN SERIES xix CHAPTER 1.1 1.2 1.3 1.4 History Preceding 1986 Technological Advances Comparison to FTIR and NIR Absorption Overview of the Book CHAPTER 2.1 2.2 2.3 2.4 MAGNITUDE OF RAMAN SCATTERING Theoretical Overview Definition of Raman Cross Section Magnitude of Raman Cross Sections Raman Scattering Intensity CHAPTER 3.1 3.2 3.3 3.4 INTRODUCTION AND SCOPE COLLECTION AND DETECTION OF RAMAN SCATTERING Signal Magnitude and Collection Function Instrumental Variables Comprising the Collection Function Spectrometer Response Function Multiplex and Multichannel Spectrometers CHAPTER SIGNAL-TO-NOISE IN RAMAN SPECTROSCOPY 4.1 Definition and Measurement of SNR 4.2 Noise Sources V I 10 12 15 1s 20 24 30 35 35 37 41 43 49 49 52 vi CONTENTS 4.3 4.4 4.5 4.6 Signal-to-Noise Ratio Expressions SNR Figure of Merit SNR and Detection Limits SNR for Multiplex Spectrometers CHAPTER 5.1 5.2 5.3 5.4 5.5 INSTRUMENTATION OVERVIEW AND SPECTROMETER PERFORMANCE Major Spectrometer Components Laser Wavelength Dispersive vs Nondispersive Spectrometers Performance Criteria Samples for Spectrometer Evaluation CHAPTER SAMPLING MODES IN RAMAN SPECTROSCOPY 6.1 Sampling Overview 6.2 6.3 6.4 6.5 6.6 6.7 Performance Criteria 180" Backscattering Geometry 90" Sampling Geometry Reducing the Laser Power Density at the Sample Path Length Enhancement Polarization Measurements CHAPTER 7.1 7.2 7.3 7.4 7.5 7.6 LASERS FOR RAMAN SPECTROSCOPY Overview Ar+ and Krt Ion Lasers Helium-Neon Lasers Neodymium-YAG (Nd:YAG) Diode Lasers Laser Wavelength Filtering CHAPTER DISPERSIVE RAMAN SPECTROMETERS 8.1 Overview 8.2 8.3 8.4 8.5 Dispersive Spectrometer Configurations Detector Considerations Single-Channel Detectors Multichannel Detectors and CCDs 61 65 67 68 73 74 75 78 79 83 95 95 97 99 114 118 120 122 127 127 130 133 134 137 142 149 149 155 179 180 183 CONTENTS 8.6 Recording Methods for Dispersive Spectrometers 8.7 Examples of Dispersive Raman Applications CHAPTER 9.1 9.2 9.3 9.4 9.5 NONDISPERSIVE RAMAN SPECTROMETERS Tunable Bandpass Filters Fourier Transform Raman Spectroscopy Multichannel Fourier Transform Raman Spectroscopy Extensions of FT-Raman for Longer Wavelength Operation FT-Raman Examples vii 203 215 221 22 225 240 245 246 CHAPTER 10 CALIBRATION AND VALIDATION 251 10.1 Overview 10.2 Frequency and Raman Shift Calibration 10.3 Instrument Response Function Calibration 10.4 Absolute Response Calibration 10.5 Summary of Calibration and Validation Procedures 25 25 269 288 289 CHAPTER 11 RAMAN MICROSCOPY AND IMAGfNG 293 11.1 11.2 11.3 11.4 293 295 309 316 Overview of Raman Microscopy Single-Point Raman Microspectroscopy Line Imaging Two-Dimensional Raman Imaging CHAPTER 12 FIBER-OPTIC RAMAN SAMPLING 333 12.1 Overview of Fiber-optic Sampling 12.2 Fiber-optic Basics 12.3 Fiber-Spectrometer Interface 12.4 Fiber-optic Probes 12.5 Comparisons of Fiber-optic Sampling Probes 12.6 Waveguide Sampling for Analytical Raman Spectroscopy 12.7 Examples of Fiber-optic Sampling 333 334 337 342 359 364 369 CHAPTER 13 RAMAN SPECTROSCOPY OF SURFACES 373 13.1 Overview 13.2 Surface Sensitivity 373 375 CONTENTS Vlll 13.3 13.4 13.5 13.6 Sampling Considerations Surface Raman Spectroscopy without Field Enhancement Electromagnetic Field Enhancement Examples of Analytical Applications INDEX 379 382 390 409 415 PREFACE This book was inspired by the transition of Raman spectroscopy from a technically demanding research technique to a useful and practical method of chemical analysis There are many fine texts and thousands of scientific articles on research in Raman spectroscopy, primarily oriented toward understanding the Raman effect itself and using Raman scattering to probe molecular structure and dynamics These research efforts have reached a high level of sophistication and have yielded valuable chemical and physical insights, but they rarely resulted in practical techniques for chemical analysis until approximately 1986 The impediments to broad applications of Raman spectroscopy to chemical analysis were mainly technological rather than fundamental The instrumentation required to observe the weak Raman effect was too cumbersome and expensive for routine analysis, and interference from fluorescence precluded application to a broad range of industrial samples As a result, the advantages of Raman spectroscopy over more common infrared absorption techniques were not exploited in analytical problems Major technological and scientific innovation in the past 10 to 15 years has significantly broadened the applicability of Raman spectroscopy, particularly in chemical analysis Fourier transform (FT)-Raman, charge-coupled device (CCD) detectors, compact spectrographs, effective laser rejection filters, near-infrared lasers, and small computers have contributed to a revolution in Raman instrumentation and made routine analytical applications possible An increase in instrumental sensitivity by factors as large as lo5, plus decreases in both interferences and noise resulted from this “revolution.” The number of vendors of Raman spectrometers increased from to 12 over a 10-year period, and integrated commercial spectrometers led to turnkey operation and robust reliability This book is intended to introduce a student or practitioner of analytical chemistry to the technical elements and practical benefits of the “Raman revolution.” It is not intended to describe “high-end” Raman techniques such as nonlinear or time-resolved Raman spectroscopy, nor does it attempt to describe the many theoretical treatments of Raman scattering The book emphasizes the concepts and technology important to applications of Raman spectroscopy in chemical analysis, with attention to calibration, performance, and sampling modes While many recent innovations in analytical Raman spectroscopy are ix X PREFACE technically sophisticated, their objectives are reliability, accuracy, reduction of interferences, and ease of operation rather than ultimate spectral resolution or sensitivity The emphasis of both the theory and instrumentation discussions in this book is the practical analysis that has resulted from recent technological developments Techniques such as nonlinear Raman (CARS, hyperRaman, stimulated Raman, etc.), picosecond transient Raman, single-crystal Raman, gas-phase Raman,and so forth are excluded not because they are unimportant, but because they currently have limited use in routine chemical analysis The audience for this book should include graduate students, practicing chemists, and Raman spectroscopists who seek information on recent instrumentation developments It is not a comprehensive review but more of a textbook intended as an introduction to modern Raman spectroscopy In most cases, the techniques discussed are available in commercially available spectrometers, and the book should be useful to chemists who are implementing Raman spectroscopy in industrial or academic laboratories Although a large number of useful Raman applications involve custom-built instrumentation, the book emphasizes configurations and components used by current vendors of integrated Raman spectrometers Since commercial spectrometers can be constructed in a variety of configurations, instruments from different manufacturers often differ significantly in applicability and performance Specific manufacturers are mentioned in the text to identify a particular approach or optical configuration Available commercial units differ widely in performance and are often optimized for particular sample types Mention of a manufacturer in the text does not imply an endorsement but may be useful to the reader in order to appreciate differences in design objectives There is no “best” manufacturer or configuration, but certain designs are more applicable to certain situations, depending on the sample and analytical objective It will become obvious that the sample dictates the choice of spectrometer type, and no single Raman system covers all possible applications The book is divided into roughly three general areas on theory and instrumentation Chapters to cover the origin and magnitude of Raman scattering and the major factors determining the signalhoise ratio Chapters to 10 discuss instrumental components and configurations and methods of calibration Chapters I to 13 address the widely studied specialty areas of Raman microscopy, fiber-optic sampling, and Raman spectroscopy of surfaces In most chapters , many examples of applications to Raman spectroscopy to analytical problems are provided Notes on Conventions The definitions of several symbols and certain conventions are not used uniformly in the Raman spectroscopy literature, and some choices were ELECTROMAGNETIC FIELD ENHANCEMENT 405 may change the metal surface, either by surface rearrangement, oxidation, or contamination For the BPE example listed in Table 13.4, high sensitivity and low detection limits were achieved for Ag island films Reproducibility of the Raman signal following exposure of Ag island films to 1.2 x lop6 M BPE was 14 per cent for annealed films and 27 per cent for unannealed films (30) The stability of the films depended on storage conditions, with good stability over extended periods when stored in a vacuum Exposure to air or water decreased SERS intensity and increased the background from contamination A promising modification of the silver island approach involves protection of the island film with a very thin layer of silica (46) The silica layer is thin enough so that molecules on its surface are still subject to field enhancement, although the chemical enhancement between silver and adsorbate is lost The silver islands are protected from adsorption of atmospheric impurities and the field enhancement is quite stable with time Silica-protected Ag island films not exhibit the large enhancements encountered with bare Ag islands or electrochemical roughening, but the decreased enhancement may be more than compensated by improved reproducibility for many analytical applications In order to enhance or modify the chemical selectivity of an SERS substrate, it is possible to chemically derivatize the metal surface For example, covalent bonding of a hydrocarbon to a silver island film should selectively adsorb nonpolar analytes from an aqueous solution The general approach is shown schematically in Figure 13.24 for the case of metal ions detected by a surfacebound complexing agent (47) Field enhancement is provided by the substrate, while adsorption selectivity results from the chemistry of the derivatized Ag islands on glass modified Ag islands \ A in water A= hydrophobic analyte Figure 13.24 Procedure for forming a self-assembled monolayer on a silver island film, followed by adsorption of a hydrophobic analyte (A) preceding SERS analysis See References 53, 57 and 58 406 RAMAN SPECTROSCOPY OF SURFACES surface Many procedures similar to that depicted in Figure 13.24 are based on self-assembled monolayers (SAMs) made when thiols adsorb irreversibly to gold, silver, or copper Quite sophisticated SERS methods exploiting both field enhancement and chemical selectivity have been devised using SAMs on vapor-deposited silver islands (47-49), electrochemically roughened metals (50,51), and silver roughened with nitric acid (5233) A further extension of SERS to silica surfaces involved alkylsilane layers bonded to mechanically roughened silver, as a model for a chromatographic stationary phase (54) 13.5.3.3 Metal Colloids Colloidal suspensions of metal particles may be formed in aqueous solution by chemical reduction of metal salts (30,55,56) The distribution of particle diameter depends on conditions but may be controlled to a range suitable for EM field enhancement An example of an aggregate of colloidal silver is shown in Figure 13.12A Colloidal suspensions of silver and gold are available commercially but most commonly are prepared shortly before use A common diagnostic of colloidal properties is a UV-Vis absorption spectrum, which depends on the particle size and density Visible light absorption by small metal particles results from interactions of the optical electric field with particles having diameters smaller than the wavelength and is related to the same phenomena that underlie EM field enhancement When used for chemical analysis, the metal colloid may be added to an analyte solution or immobilized before exposure to the analyte Metal colloids have been studied extensively with visible (2,57,58) and NIR excitation (59,60) Depending on conditions, many procedures for preparing colloids lead to particles that are too small to exhibit large field enhancement, so an aggregating agent is sometimes added to produce structures similar to that shown in Figure 13.12A Much of the variability in Raman intensities observed with colloids is due to variations in the agglomeration time and procedure Jones and co-workers compared several agglomeration procedures to determine the reproducibility of the SERS enhancement of resonant Raman dyes based on azobenzotriazole (61) The agglomerating agent was either added to the colloid-dye solution or the colloid was preagglomerated before adding the dye The Raman intensities were monitored as a function of agglomeration time, and the standard deviation was determined for repeated spectra and colloid preparations Typical spectra are shown in Figure 13.25 for three concentrations of an azo dye adsorbed on an agglomerated silver colloid The combination of surface and resonance enhancement yields high SNR spectra in the concentration range of lo-' to M A calibration curve of intensity vs dye concentration was linear (Fig 13.26) over about orders of magnitude but leveled off sharply when the dye formed a monolayer on the colloid 407 ELECTROMAGNETIC FIELD ENHANCEMENT 4-(5 '-azobenzotriazol)-aminonapthalene on silver colloid 6.7 x 10-5M 6.7 x 6.7 x 900 M M 1800 1350 Raman shift, cm-' Figure 13.25 Surface-enhanced resonance Raman spectra (SERS) of an azo dye adsorbed on silver colloids from solutions of the indicated concentrations; 514.5 nm, 25 mW, 10 sec integrations on CCDldispersive Raman microscope (Renishaw) (Adapted from Reference 61.) I 8c * Y 4-( '-azobenzotriazol)-aminonapthalene on silver colloid molar concentration in solution Figure 13.26 Calibration curve based on spectra similar to those in Figure 13.25 (Adapted from Reference 61 with permission.) 408 RAMAN SPECTROSCOPY OF SURFACES surface The most sensitive Raman spectroscopy ever reported uses SERS on silver colloids to detect single molecules of adsorbed dyes (60,62,63) Reproducibility of the Raman intensity for the 1424 cm-' band of a particular dye at 10@ M is summarized in Table 13.5 Relative standard deviations of five spectra of a given colloid solution as well as the relative standard deviation (rsd) for five different colloid solutions are listed Significantly higher (rsd) was observed for different colloid solutions, implying that the particle agglomeration and analyte adsorption are quite sensitive to uncontrolled changes in conditions The authors (6 1) concluded that preaggregation with poly-(L-lysine) yields quite reproducible intensities (rsd